Datasheet

LM134, LM234, LM334
SNVS746C –MAY 2004–REVISED MARCH 2005 www.ti.com
Figure 7. Zero Tempco Current Source
The set current (I
SET
) is the sum of I
1
and I
2
, each contributing approximately 50% of the set current, and I
BIAS
.
I
BIAS
is usually included in the I
1
term by increasing the V
R
value used for calculations by 5.9%. (See
CALCULATING R
SET
.)
(5)
The first step is to minimize the tempco of the circuit, using the following equations. An example is given using a
value of +227μV/°C as the tempco of the LM134 (which includes the I
BIAS
component), and −2.5 mV/°C as the
tempco of the diode (for best results, this value should be directly measured or obtained from the manufacturer
of the diode).
(6)
(7)
With the R
1
to R
2
ratio determined, values for R
1
and R
2
should be determined to give the desired set current.
The formula for calculating the set current at T = 25°C is shown below, followed by an example that assumes the
forward voltage drop across the diode (V
D
) is 0.6V, the voltage across R
1
is 67.7mV (64 mV + 5.9% to account
for I
BIAS
), and R
2
/R
1
= 10 (from the previous calculations).
(8)
This circuit will eliminate most of the LM134's temperature coefficient, and it does a good job even if the
estimates of the diode's characteristics are not accurate (as the following example will show). For lowest tempco
with a specific diode at the desired I
SET
, however, the circuit should be built and tested over temperature. If the
measured tempco of I
SET
is positive, R
2
should be reduced. If the resulting tempco is negative, R
2
should be
increased. The recommended diode for use in this circuit is the 1N457 because its tempco is centered at 11
times the tempco of the LM134, allowing R
2
= 10 R
1
. You can also use this circuit to create a current source with
non-zero tempcos by setting the tempco component of the tempco equation to the desired value instead of 0.
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